1. Field of the Invention
The present invention relates to a photonic bandgap fiber (hereinafter, abbreviated as “PBGF”), and more particularly, relates to an extended triangular lattice (ETL) type PBGF which has a wider transmission bandwidth and is available to optical transmission and fiber laser optical transmission from ultraviolet range to visible light range, near-infrared range and far-infrared range.
2. Description of the Related Art
The present inventors developed an extended triangular lattice type PBGF having capillary cores, as disclosed in K. Takenaga, N. Guan, R. Goto, S. Matsuo, K. Himeno, “A New Photonic Bandgap Fibre with Extended Triangular Lattice and Capillary Core”, ECOC 2005 Proceedings-Vol. 2 Paper Tu 1.4.2. FIGS. 1 and 2 are diagrams illustrating the arrangement of the conventional extended triangular lattice type PBGFs 1 and 6.
The extended triangular lattice type PBGF 1 shown in FIG. 1, includes a capillary core 4 in which a plurality of hole 2 are regularly provided in the longitudinal direction of an optical fiber, made of silica glass and in cross section of the fiber seven holes 2 in the center of the fiber are compactly arranged in a triangular lattice shape. Further, the extended triangular lattice type PBGF 1 includes a cladding 5 around the core 4 in which holes 2 and a silica glass part 3 are arranged in an extended triangular lattice shape.
The extended triangular lattice type PBGF 6 shown FIG. 2, includes a capillary core 7 in which in total thirty seven holes 2 are compactly arranged in a triangular lattice shape. Specifically, one hole of the thirty seven holes is arranged in the center of the fiber, six holes of the thirty seven holes are arranged in the first layer surrounding the one hole, twelve holes of the thirty seven holes are arranged in the second layer surrounding the first layer, and eighteen holes of the thirty seven holes are arranged in the third layer surrounding the second layer. In addition, the extended triangular lattice type PBGF 6 includes a cladding 5 around the capillary core 7 in which holes 2 and a silica glass part 3 are arranged in an extended triangular lattice shape.
In these extended triangular lattice type PBGFs 1 and 6, holes 2 of the capillary core 4, 7 and holes 2 of the cladding 5 have the same hole diameter. The cross section of holes 2 can be either a circular shape or a hexagonal shape, depending on the manufacturing condition of the PBGF, for example such as pressure difference between the interior of the hole and the exterior of the fiber base material during the course of drawing the fiber, temperature, etc.
The extended triangular lattice type PBGF, described in literature of K. Takenaga et al., employs capillary cores and thus allows broadly extending the transmission bandwidth, as compared to a conventional PBGF using hole core (air core).
However, it is expected that the application field of the optical fiber will be increasingly extended and the range of transmission bandwidth required in an optical fiber will gradually become wider from now on. Therefore, there is a need for providing an optical fiber having wide transmission bandwidth.